Profound immunosuppression as occurs in HIV-infected subjects is frequently associated with complicating mycobacterial infections. Mycobacterium tuberculosis is a worldwide pathogen and is a major cause of morbidity and mortality in both normal and immunosuppressed subjects. Non-tuberculous mycobacteria such as M. avium can be opportunistic infections that infect immunosuppressed individuals or subjects with underlying disorders such as pulmonary silicosis. Regardless of whether mycobacteria are acting as pathogens or opportunistic infectious organisms, mycobacteria in the lung first infect alveolar macrophages (AMs), the resident inflammatory cell of the alveolar spaces. AMs must be primed and activated by cytokines such as IFN-gamma or TNF-alpha to maximally respond to infectious organisms such as mycobacteria. Although it is recognized that AMs are central to the pathogenesis of mycobacterial disease, there are few studies that have examined the role of AMs in vivo in response to mycobacterial infection. We have developed a novel approach to "reconstitute" normal and activated AMs into the lungs of immunodeficient animals. We will use this new approach to test the hypothesis Deficiencies in the response of AMs to mycobacteria such as MAC or M. tuberculosis permit initial lung infection and subsequent dissemination during immunosuppression; conversely, correction of these AM deficiencies will restore alveolar immunity, control lung infection and prevent dissemination. We will examine the underlying mechanisms by which AMs respond in vivo to mycobacterial infection and will then use a variety of strategies to activate AMs for reconstitution to see if alveolar host defense is restored and infection eradicated. This will also test whether AMs activated by pro-inflammatory cytokines such as IFN-gamma mediate alveolar host defense to mycobacteria by AM-derived TNF-alpha. These Specific Aims include: 1) to determine the mechanisms by which normal AMs reconstituted into the lungs of immunodeficient mice restore alveolar host defense to mycobacteria and prevent dissemination, 2) to determine if proinflammatory cytokines such as IFN-gamma are essential for alveolar host defense to mycobacteria and to prevent dissemination, 3) to determine if ex vivo gene therapy to reconstituted macrophages results in persistent overexpression of pro-inflammatory cytokines such as IFN-gamma in vivo and improves alveolar host defenses to mycobacteria and prevents dissemination, and 4) to determine if the effects of pro-inflammatory cytokines such as IFN-? on alveolar host defenses to mycobacteria are mediated by AM-derived TNF-alpha. This proposal will test hypotheses in vivo not possible by other means and will determine whether reconstitution of normal or activated AMs is sufficient to restore alveolar host defense to mycobacterial disease despite the presence of ongoing systemic immunosuppression.